Frequency control systems for vibratory transducer



May 15, 1956 Filed April 23, 1953 G. M PHERSON, JR

FREQUENCY CONTROL SYSTEM FOR VIBRATORY TRANSDUCER 2 Sheets-Sheet 1 32,Strain Gage Governor I 2 Prime MoIVer I on I75 I 25? String I750 I I I I532 533 i P Carrier Motor Fine" Current Control G Receiver -Amplifier o254 255 260 Drill Collar 0 CD 3 8-80 .2 "I20 I' I r L "I60 I? 'u a Fig 3.9 C BI O a: I 2.. l I O I N I 253 25l e30 I I phase Carrier I DI ICurrent y I ec or Transmitter I i I 252 I I 4 I 25o I l I Amplifier L lI I0, Bit

May 1956 G. MCPHERSON, JR

FREQUENCY CONTROL SYSTEM FOR VIBRATORY TRANSDUCER 2 Sheets-Sheet 2 FiledApril 25, 1953 |5,- Drill Collar l2, Mognefos'rricfive Motor IO, an

EE E 22 8 United States Patent FREQUENCY CONTROL SYSTEMS FOR VIBRATORYTRANSDUCER George McPherson, Jr., Columbus, Ohio, assignor, by mesneassignments, to Drilling Research, lino, Houston, Tex., a corporation ofDelaware Application April 23, 1953, Serial No. 35%,732 16 Claims. (Cl.318-118) This invention re.ates to frequency control systems,particularly to frequency control systems, preferably automatic, thatare particularly adaptable and especially useful in vibratorywell-drilling apparatus to maintain substantially optimum frequency ofoperation therein during varying conditions of drilling to enable thedrilling equipment to attain the desired depth of the well at minimumcost and in minimum time.

The frequency control systems of this invention are particularly usefulin well-drilling systems of the type described and claimed in thecopending application for U. S. Letters Patent of Boyd A. Wise et al.,Serial No. 350,314. Such a drilling system includes a drill stringhaving means for applying a static load to a drill bit secured to thelower end of a vibratory column at the lower end of a drill string, thecolumn including an elongated magnetostrictive transducer. In thepresent invention, means are provided for supplying an alternatingcurrent to the transducer to produce a changing flux of high peakdensity therein and further means are provided responsive to measurablecharacteristics in the vibratory column, such as the strain at asuitable preselected location in the vibratory column and the resistivecomponent of the impedance of the transducer, that may vary according todeviation from the resonant frequency, including means responsive to thealternating current supplied to the transducer, for controlling thefrequency of excitation of the transducer to maintain the vibration ofthe column at substantially the resonant frequency of the column whilein engagement with strata to be penetrated.

The objects and advantages of the present invention can be understoodmost fully in connection with an explanation of various problems andlimitations involved in the drilling of deep wells. For economicreasons, deep wells, which may extend several miles underground, arenecessarily of small diameter, the size of the casing ranging from aboutfive inches to about inches, the more usual size being from six to teninches. The severe limitations of hole size require a motor typeparticularly suited to the application, if adequate forces and power areto be produced. Magnetostrictive motors of a specialized type, such asare shown in the copending application of Wise et al. mentioned above,meet the necessary limitations and requirements, and are especially wellsuited to the drilling of wells.

The severity of the limitations imposed by the small size of the borehole is intensified as the drilling rate increases. As the bitpenetrates more rapidly, the chips or fragments of the strata must beremoved at higher rates, and it is necessary to increase the circulationof mud, a term generally used to refer to the medium that carries awaythe drilling chips. The drill string and the drilling column must, ofcourse, provide a flow passage for the circulation of mud at reasonablepumping pressures and in sufficient volume to remove the chips as fastas they are produced, and thus maintain the drill in contact with thestrata to be penetrated and unimpeded in its progress by any substantialdepth of chips or fragments.

To provide maximum rate of penetration with a given drilling system, itis necessary to operate the vibratory column substantially at resonance.For this reason, the vibratory column preferably has a length ofapproximately one-half wave length or an integral multiple thereof at afrequency near the middle of a band of operating frequencies availablewith a particular alternating current generator furnishing power to thetransducer. Different conditions of drilling, as may arise fromdifferent types of rock formation to be penetrated, cause the naturalresonant frequency of the vibrating column to vary, and it is necessaryto be able to vary the operating frequency accordingly as changingconditions arise, in order to maintain operation substantially atresonance.

It is a primary object of the present invention, therefore, to providefrequency control systems, preferably automatic, for varying theoperating frequency in vibratory drilling systems wherein the resonantfrequency of a vibratory column may vary with changes in drillingconditions.

Because of the requirements that the drill string shall have an open,unobstructed passage for flow of drilling mud, available space forelectrical conductors is at a premium. In accordance with the presentinvention, no extra conductors are required. Nevertheless, the controlsystem responds to vary the frequency in direction and extent tomaintain vibration of the transducer at or near maximum amplitude forhigh rates of drilling. In one modification, auxiliary conductors, ifused, may be quite small or they may be eliminated by usingcarrier-current systems.

In a preferred form of the invention, advantage is taken of the factthat the resistive component of the impedance of the transducer changeswith departure of operation of the transducer from its resonantfrequency. By maintaining constant, or relatively so, the currentsupplied to the transducer winding, the frequency of maximum power willcoincide with the frequency of maximum resistance. The frequency of thealternating-current power supply may be varied to maintain maximum powerdelivery to the transducer in response to changes in power as determinedat the surface. In accordance with the invention, the speed of thegenerator is cyclically varied, first in one direction and then in theother direction to vary its frequency. The change in power with suchcyclical changes in frequency is detected or observed. The cyclicalvariation is then modified so that a greater change in freqency occursin the direction that increases the power output of the generator. Withthe greater change in the power-increasing direction, the generatorfrequency is varied so as to reach the frequency that provides maximumpower output to the transducer.

In another form of the invention, the frequency of excitation of thetransducer is controlled so as to maintain the vibration of thevibratory column in the drilling system at substantially the resonantfrequency by means responsive to the phase relationship between thecurrent supplied to the transducer and the strain at a suitablepreselected location in the vibratory column. A preferred location fromwhich to obtain a strain signal is at approximately the partial velocitynode in the vibratory column. Another suitable location is at the bit.Using either of these locations, means are provided for detecting thephase relationship between the current and the strain and for varyingthe operating frequency in such manner as to maintain substantially aphase relationship between the current and the strain, such phaserelationship being a characteristic of substantially resonant operation.

It is a further object of this invention to provide frequency controlsystems having the features and advantages summarized in the foregoingparagraphs. Further objects and advantages will be apparent from thefollowing detailed description and the claims.

In the drawings:

Fig. 1.is aschematic view illustrating a frequency control systemaccording to the present invention;

Fig. 2 is a graph explanatory of the theory of operation of thefrequency control system of Fig. 1;

Fig. 3 is another graph similar to Fig. 2;

Fig. 4 is a schematic view diagrammatically illustrating another form offrequency control system according to the present invention; and

Fig. 5 is a graph explanatory of the operation of the system of Fig. 4.

In order that operation at substantially the optimum or resonantfrequency may be maintained in a vibratory drilling system despite thefact that the resonant frequency changes during drilling, a frequencycontrol system should be provided to respond to the output of thetransducer in such way as to regulate the frequency of the, alternatingcurrent power in accordance with the changing resonant frequency of thetransducer. Any one of several variables whose magnitudes change withdeparture of transducer operation from its resonant frequency, such asstrain-gauge output, may be availed of in controlling frequency. Onefrequency control method of the present invention comprises regulatingthe frequency of the alternating-current power supply either manually orautomatically in response to phase change as between the alternatingcurrent supplied to the transducer and the transducer strain in thevicinity of the bit, or preferably in the region of the partial node,where the waveform of the strain signal is more nearly sinusoidal. As apart of the present invention, it has been found that at resonance aphase difference of approximately ninety degrees exists between thealternating current and the transducer strain. Thus, as this phase anglechanges from ninety degrees, the frequency of the alternating currentgenerator or alternator should be changed in the direction to restorethe ninety-degree relationship.

Fig. 1 schematically illustrates a preferred system for providing suchcontrol. The strain gauge 32 may be either at the bit or at the partialnode, as illustrated. The output from the strain gauge 32 is applied toan amplifier 250 whose output is fed to the input terminals of one inputsection of a phase-sensitive detector 251. A current signal derived froma low resistance 252 inserted in the supply line 24 is applied,preferably through a high-capacitance blocking condenser 253, to theinput terminals of the other input section of the phasesensitivedetector 251. The blocking condenser 253 is needed where, as in thecircuit of Fig. l, a direct-current component of power is supplied tothe transducer as by the direct-current generator 168 driven by theprime mover 175a and connected through the inductance 167 to thetransmission lines 23, 24, in addition to the alternating-currentcomponent of power supplied by the alternating-current generator 162driven by the prime mover 175 and connected through the capacitor 164 tothe transmission lines 23, 24.

The phase-sensitive detector 251, preferably of the electronic typeknown in the art, provides at its output circuit 259 a direct-currentpotential approximately proportional to any variation from aninety-degree phase relationship between the strain signal received fromthe strain gauge 32 and the current signal received from the lowresistance 252 (which is unchanged in phase by the capacitor 253). Thisoutput potential has one polarity When the phase relationship is greaterthan ninety degrees and has the opposite polarity when the phaserelationship is less than ninety degrees. This output potential isapplied to the input terminals of a motor control amplifier 254 whichprovides an output of sufficient amplitude to operate a motor 255. Thedirection in which the motor 255 rotates is dependent upon the polarityof the voltage applied to it from the output of the motor-controlamplifier 254. It will be positive or negative in accordance with thepolarity of the output of the phase-sensitive detector 251.

When the phase relationship between the strain signal from the straingauge 32 and the current signal from the low resistance 252 is ninetydegrees, the output of the phase-sensitive detector 251 is zero, so themotor 255 receives zero voltage from the motor-control amplifier 254 andremains at a standstill. As the phase angle decreases, however, as bychange in the phase of the strain or output of strain gauge 32, thephase-sensitive detector 251 provides an output voltage of one polarity,which, as amplified by the motor-control amplifier 254 and applied tothe motor 255, causes the motor to rotate in one direction, a directionwhich through suitable gearing, as indicated by broken line 257 adjustsa governor 258 to change the speed of the prime mover in the direction,and by an amount, which restores .the ninety-degree relationship. Achange in the phase of the strain in the opposite direction reverses thepolarity of the output of the phase-sensitive detector 251, therebyreversing the direction of rotation of the motor 255, again to restorethe ninety-degree relationship.

Considerable attenuation and phase shift may be present in transmittingthe strain signal from the transducer to the top of a long drill stringbecause of the characteristics of the line used to transmit this signal.The power transmission line may not be used for transmitting theunmodified strain signal since the strain signal is of the samefrequency as that of the very much greater power-supply voltage.

To avoid this deterioration in the strain signal, and to avoid theeffect of phase shift in the transmission line which may occur betweenthe low resistance 252 and the transducer 12, it is preferred that asub-assembly including the resistor 252, the strain signal amplifier250, the phase-sensitive detector 251 and a high-frequencycarrier-current transmitter 530 be located in a container 531 insertedin the drill string 21 just above the mechanical filter 20, as in thelower end of a heavy drill collar 15 forming a part of the static loadto the bit 10. The output signal from the phase-sensitive detector 251is then used to modulate the carrier-current produced by its transmitter530. The modulated carrier signal, derived from the carrier-currenttransmitter 530, is transmitted upward along the power transmission line23, 24, and after passage through a filter 532 in the line isdemodulated by a carrier-current receiver 533 for development of adirect-current potential that is proportional to the output of thephase-sensitive detector 251, and whose polarity changes in the samemanner as that of the output of said detector. This potential, appliedto the input of the motor control amplifier 254, controls the speed anddirection of the motor 255 which in turn controls the speed of thealternator, and thus the frequency, to maintain the ninety-degree phaserelationship between the current and the strain signal.

Of course, the governor 258 may be manually adjusted as by a wheel 259in accordance with deflection from ninety degrees of the phase-anglemeter 260. In general, the meter 260 and provisions for manual controlare provided even with a fully automatic system.

The soundness of the basis for the control system thus far described hasbeen experimentally verified. In Fig. 2, the graph 261 illustrates thechange in phase angle between the strain at the bit 10 and thealternating current supplied to the transducer. The graph 262 represents the strain at the bit. As the frequency increases, it will beseen that the phase angle decreases from about one hundred and fortydegrees, passing through ninety degrees as the strain at the nodeapproaches a maximum. The phase angle from about one hundred degrees toabout forty degrees rapidly decreases and then more slowly decreases.The maximum strain occurs at a phase angle of about ninety degreeswithin the limits of accuracy of the measurements.

"The graphs 263 and 264 of Fig. 3, respectively, correspond to those ofFig. 2 but the graph 263 represents phase angle between the alternatingcurrent supplied to winding 18 and the output of strain gauge 32 locatedat the nodal point or partial velocity node, as in Fig. 1. The angleshave a minus sign since the strain at the partial velocity node is onehundred and eighty degrees out of phase with the strain at the bit 10.The region of maximum strain occurs, graph 264, in the region of ninetydegrees.

The steepness of the change of phase angle of both Figs. 2 and 3 is afunction of the Q of the transducer. It can be shown that Q equals theproduct of one-half the resonant frequency times the rate of change ofphase with respect to the frequency at resonance. The higher the Q, thesteeper will be the slope of graphs 261 and 263 in the region of ninetydegrees. Thus, while band width at the resonant frequency is ecreasedcorresponding to an increase in Q, the sensitivity of control isincreased and the control system of Fig. 1 will function to maintain thetransducer operating at its resonant frequency. For a decreased Q, thesensitivity of control is decreased but, for this case, the frequency ofoperation need not be controlled to Within as close limits as for higherQ operation because of the increased band width. Thus, this automaticcontrol system will function satisfactorly over a wide range of Qvalues.

Further in accordance with the invention, advantage has been taken ofthe fact that the resistive component of the transducer impedance hasbeen found to be a maximum at a frequency not far removed from theresonant frequency of the vibratory column. By reason of that fact, andof a known relationship in electrical theory, a control system of adifferent type may be utilized, such as shown in Fig. 4-. This secondform of control system obviates the necessity of obtaining any signalsfrom downhole sensing elements. The system functions in a manner toproduce maximum power input to the transducer with a substantiallyconstant current supplied thereto. With power output determined by theproduct of the circuit resistance and the square of the current, hightransducer output will be maintained by adjusting the frequency inresponse to change in the resistive component of the transducerimpedance. The control action is in a direction to raise or lower thefrequency as needed to maintain the resistive component of thetransducer impedance at a value approaching its maximum.

In its simplest form, and with the current held constant by any suitablemeans as by any well-known automatic constant current-control circuitoperating on the exciter system, the alternator frequency is varied bychanging the governor setting as by the adjusting wheel 259, shown inFig. l, to maintain a maximum reading of a wattmeter 590 responsive tothe power transmitted to the transducer through conductors 23 and 24.

Attached to the shaft of the wattmeter 5% is a contact arm 501 movablebetween a mechanical bumper 562 and an electrical contact 563 for use inproviding automatic frequency control. The bumper and electrical contact503 are mounted on a light-weight vane 5454 which is pivoted on, androtatable relative to, the meter shaft which carries the pointer and thecontact element 501. Accordingly, when contact arm 5% is rotated in aclockwise direction during periods of increasing power, the contact 561ais moved against the contact 563. Arm 501 then moves vane 5134- in aclockwise direction maintaining closed a circuit through contacts 591aand During periods when the power output of alternator 162 is decreasingthe contact arm 5&1 first opens said circuit and then engages the bumper5% to move the vane 504 in a counterclockwise direction. The contact arm501 may, if desired, serve also as the indicating needle of thewattmeter 5%.

The manner in which the relative movement between the contact arm 501and the contact-carrying vane 504 automatically adjusts the governor 258to maintain maximum power flow in lines 23 and 24 will now be explained.The armature of motor 255 which adjusts the setting of the governor 258is energized from any suitable source of direct voltage 508 by way ofslip rings 509 and 510 and direction-controlling commutator contactsegments 511 and 512. As shown, the motor 255 with its field winding255a, energized from a suitable voltage source, will rotate at low speedin one direction. It is rotated at low speed by reason of the inclusionin the motor circuit of a speed-reducing resistor 513. When thepositions of contact 511 and 512 are reversed, the direction of rotationof motor 255 is, of course, reversed.

By driving the segments 511 and 512 at relatively low speed, as by amotor 514 of the type having a geareddown output shaft, the motor 255adjusts the governor 258 first in a direction to increase the speed ofthe prime mover and then in a direction to decrease its speed. Theextent of the adjustment is small and is insuificient to make more thana slight change in frequency of the alternator 162. Sufficient change,however, is made in the frequency for the response of the wattmeter 500to indicate whether or not increased power results from an increase infrequency or whether decreased power results therefrom.

It will now be assumed that the motor 255 is rotating in a direction toincrease the frequency of alternator 162 and that the power throughlines 23 and 24 to the transducer is increasing. Accordingly, a circuitwill be completed from one side of a voltage source 515 through anoperating coil 516 for a relay contact 517 and through the contacts 501aand 503 to the other side of the voltage source 515. The relay contact517 closes to remove the resistor 513 from the motor control circuit asby placing a short circuit around it. The effect of short-circuiting theresistor 513 is to increase the speed of the motor 255. Thus, whenincreasing frequency results in an increase in power, the motor 255 runsfaster in the direction to increase the frequency of the alternator 162.As soon as the positions of the commutator contact segments 511 and 512are interchanged from their positions as illustrated, however, the motor255 is energized for reverse rotation. The resultant reduction infrequency of the alternator 162 reduces the power in lines 23 and 24 andthe wattmeter 500 immediately responds to move contact arm 501 to openthe circuit of relay coil 516. Accordingly, the resistor 513 iseffectively inserted in the motor circuit to slow down the motor and toproduce a slower speed of operation in reducing the frequency thanoccurred in its operation to increase the frequency.

The system responds to develop maximum power output. The power isincreased by the rising frequency and the vane 504 is moved in aclockwise direction by contact 591a but is not moved in thecounterclockwise direction until after the opening of the circuit andnot until contact member 501 engages the bumper 502.

The graphs 520 and 521 of Fig. 45, plotted with frequency as abscissaeand respectively with power and timer cycles as ordinates, are helpfulin understanding the operation of the control system. The graph 520 isexemplary of the relationship between power to the transducer 12 andfrequency.

Plotted in arbitrary units with f0 corresponding to the frequency atwhich maximum power is obtained, graph 52.1 illustrates the variationsin frequency produced by the control system as it adjusts the frequencyof the alternator for generation of maximum power. It has been assumedthat the frequency ft is about 7 /2 per cent below the frequency in formaximum power. The graph 521 illustrates operation with resistor 513having a value such that the motor speed will be doubled when it isremoved from the motor circuit.

It is seen from graph 521 that about six timer cycles are required forthe system to adjust the alternator 162 for the development of maximumpower. More particularly, it is observed that the motor 255, whenoperating to adjust the frequency as indicated by the segments 521a,521b, etc., increases the frequency to a greater extent than oppositerotation of the motor decreases the frequency. The latter is illustratedby the segments 521 and 521", etc. For the case where the frequency istoo high, the system functions to reduce it. The graph would be amirror-image of graph 521 located to the right of the broken line drawnat ft).

The contact 503 and the bumper 502 may be adjusted on the vane 504 tovary their separation distances. Ordinarily such adjustment is made onlywhen the system is first placed into operation, to provide thesensitivity needed for stable control of a particular installation. Thegraph 521 is to be considered only as exemplary of the operation of onesystem, since with different contact positions the adjustments ofgenerator frequency in increasing and decreasing directions may varysubstantially from those shown. The number of timer cycles required forthe system to reach equilibrium may in some cases be less than the sixwhich have been illustrated and in other cases may be more.

It will be understood, of course, that, while the forms of the inventionherein shown and described constitute preferred embodiments of theinvention, it is not intended herein to illustrate all of the possibleequivalent forms or ramifications of the invention. It will also beunderstood that the words used are words of description rather than oflimitation, and that various changes, such as changes in shape, relativesize, and arrangement of parts, may be substituted without departingfrom the spirit or scope of the invention herein disclosed.

What is claimed is:

1. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an elongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein, and means responsive to measurablecharacteristics in said vibratory column that may vary according todeviation from the resonant frequency, including means responsive to thealternating current supplied to said transducer, for con-- trolling thefrequency of excitation of said transducer to maintain the vibration ofsaid column at substantially the resonant frequency of said column whilein engagement with strata to be penetrated.

2. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an elongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein and means responsive to the phase relationshipbetween the current supplied to said transducer and the strain at asuitable preselected location in said vibratory column for controllingthe frequency of excitation of said transducer to maintain the vibrationof said column at substantially the resonant frequency of said columnwhile in engagement with strata to be penetrated.

3. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an elongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein and means responsive to the phase relationshipbetween the current supplied to said transducer and the strain atapproximately the partial velocity node in said vibratory column forcontrolling the frequency of excitation of said transducer to maintainthe vibration of said column at substantially 3 the resonant frequencyof said column while in engage ment with strata to be penetrated.

4. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an elongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein and means responsive to the phase relationshipbetween the current supplied to said tranducer and the strain at saidbit for controlling the frequency of excitation of said transducer tomaintain the vibration of said column at substantially the resonantfrequency of said column while in engagement with strata to bepenetrated.

5. In a vibratory drilling system including an alternating currentsource connected to energize a vibratory column includin amagnetostrictive transducer, the resonant frequency of said vibratorycolumn being variable with changes in drilling conditions, an automaticfrequency control system comprising means for measuring the currentsupplied by said alternating current source to said transducer, meansfor measuring the strain at approximately a partial node in saidvibratory column, means for detecting the phase relationship betweensaid current and said strain and for providing a direct-current outputof one polarity when said phase relationship is greater than degrees, adirect-current output of opposite polarity when said phase relationshipis less than 90 degrees, and zero output when said phase relationship is90 degrees, and means responsive to said output for controlling thefrequency of the current supplied by said alternating-current source tomaintain said current and said strain in substantially 90-degree phaserelationship.

6. In a vibratory drilling system including an alternating-currentsource connected to energize a vibratory column including amagnetrostrictive transducer, the resonant frequency of said vibratorycolumn being variable with changes in drilling conditions: an automaticfrequency control system comprising means responsive to the currentsupplied by said alternating-current source to said transducer forproviding a current signal to an input section of a phase-sensitivedetector, strain-gauge means located at approximately a partial node insaid vibratory column for providing a strain signal to an input sectionof said phase-sensitive detector, means forming a portion of saidphase-sensitive detector for providing a direct-current output of onepolarity when the phase relationship between said current signal andsaid strain signal is greater than 90 degrees, a direct-current outputof opposite polarity when said phase relationship is less than 90degrees and Zero output when said phase relationship is 90 degrees, andmeans responsive to said output for regulating a speed-control governorof a prime mover driving said alternating-current source to control thefrequency of the current supplied by said alternatingcurrent source insuch manner as to maintain said current signal and said strain signal insubstantially 90-degree phase relationship.

7. In a Well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an ciongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein and means responsive to the resistivecomponent of the impedance of said transducer for controlling thefrequency of excitation of said transducer to maintain the vibration ofsaid column at substantially the resonant frequency of said column whilein engagement with strata to be penetrated.

8. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit se-' cured to the lower end of avibratory column at the lower end of said drill string, said columnincluding an elongated magnetostrictive transducer: means for supplyingan alternating current to said transducer to produce a changing flux ofhigh peak density therein and means responsive to the resistivecomponent of the impedance of said transducer for controlling thefrequency of excitation of said transducer to maintain the vibration ofsaid column at substantially the resonant frequency of said column whilein engagement with strata to be penetrated, including means formaintaining constant alternating current amplitude supplied to saidtransducer and means for varying the frequency of said current toprovide maximum power input to said transducer.

9. In a well-drilling system comprising a drill string having means forapplying a static load to a drill bit secured to the lower end of avibratory column at the lower end of .said drill string, said columnincluding an elongated magnetostrictive transducer: a power generatingsystem for supplying power to said transducer including an alternatingcurrent generator for supplying alternating current of constantamplitude to said transducer, driving means for said generator, andmeans responsive to change in the power output of said generator forvarying the speed of said driving means until said generator deliverspower to said transducer at maximum value.

10. In a vibratory drilling system, including an alternating-currentgenerator connected to energize a vibratory column including amagnetostrictive transducer, the resonant frequency of said vibratorycolumn being variable with changes in drilling conditions: an automaticfrequency control system comprising means including saidalternating-current generator for supplying alternating current atsubstantially constant amplitude to said transducer, driving means forsaid generator, speed-control governor means for said driving means,means for cyclically regulating said governor means to vary the speed ofsaid driving means, and thereby to regulate the frequency of thealternating current supplied by said generator to said transducer, firstin one direction then in the opposite direction, wattmeter-type meansresponsive to any increase or decrease in the power output of saidgenerator including electrical switching means to control said cyclicoperation of said driving means in such manner as to increase thevariation of said frequency in the direction that increases said poweroutput so as to exceed the variation in frequency in the oppositedirection, when at the midfrequency about which said variations areproduced the power output of said generator is measurably below theoutput of said generator at the frequency of maximum output.

11. In a vibratory drilling system, including an alternating-currentgenerator connected to energize a vibratory column including amagnetostrictive transducer, the resonant frequency of said vibratorycolumn being variable with changes in drilling conditions: an automaticfrequency control system combining means including saidalternating-current generator for supplying alternating current atsubstantially constant amplitude to said transducer, driving means forsaid generator, speed-control governor means for said driving means,means for cyclically regulating said governor means to vary the speed ofsaid driving means, and thereby to regulate the frequency of thealternating current supplied by said generator to said transducer, firstin one direction then in the opposite direction, wattmeter-type meansincluding a movable contact arm associated with a movable vane havingafiixed thereto a bumper member and an electrical contact oppositelydisposed about said movable contact arm and responsive to any increaseor decrease in the power output of said generator in such manner as toclose an electrical circuit energizing a relay for actuating electricalswitching means for shorting across said resistance upon the occurrenceof a measurable increase in power during a portion of agovernor-regulating cycle, to control said cyclic operation of saiddriving means in such manner as to increase the rate, and thereby themagnitude, of variation of said frequency in the direction thatincreases said power output so as to exceed the variation in frequencyin the opposite direction, when at the midfrequency about which saidvariations are produced the power output of said generator is measurablybelow the output of said generator at the frequency of maximum output.

12. In a vibratory drilling system, including an alternating-currentgenerator connected to energize a vibratory column including amagnetostrictive transducer, the resonant frequency of said vibratorycolumn being variable with changes in drilling conditions: an automaticfrequency control system comprising means including saidalternating-current generator for supplying alternating current atsubstantially constant amplitude to said transducer, driving means forsaid generator, speedcontrol governor means for said driving means,means including a source of direct voltage connected through rotatingcommutator means for cyclically reversing the polarity of the voltagereceived from said source and connected through a resistance to thearmature winding of a direct-current motor connected to control thesetting of said governor for cyclically regulating said governor meansto vary the speed of said driving means, and thereby to regulate thefrequency of the alternating current supplied by said generator to saidtransducer, first in one direction then in the opposite direction,wattmeter-type means including a movable contact arm associated with amovable vane having afiixed thereto a bumper member and an electricalcontact oppositely disposed about said movable contact arm andresponsive to any increase or decrease in the power output of saidgenerator in such manner as to close an electrical circuit energizing arelay for actuating electrical switching means for shorting across saidresistance upon the occurrence of a measurable increase in power duringa portion of a governor-i regulating cycle, to control said cyclicoperation of said driving means in such manner as to increase the rate,and thereby the magnitude, of variation of said frequency in thedirection that increases said power output so as to exceed the variationin frequency in the opposite direction, when at the midfrequency aboutwhich said variations are produced the power output of said generator ismeasurably below the output of said generator at the frequency ofmaximum output.

13. In combination, an electromechanical transducer whose resistancecomponent of electrical impedance is a maximum at a frequencyapproximating the mechanical resonant frequency of the transducer, meansincluding an alternating-current generator for supplying alternatingcurrent of constant amplitude to said transducer, driving means for saidgenerator, and means responsive to change in the power output of saidgenerator for varying the speed of said driving means until saidgenerator delivers power to said transducer at maximum value.

14. In combination, an electromechanical transducer whose resistancecomponent of electrical impedance is a maximum at a frequencyapproximating the mechanical resonant frequency of the transducer, meansincluding an alternating-current generator for supplying alternatingcurrent at substantially constant amplitude to said transducer, drivingmeans for said generator, means for cyclically adjusting said drivingmeans to vary the frequency of said generator first in one direction andthen in the opposite direction, means responsive to change in the poweroutput of said generator for modifying said cyclic operation of saiddriving means to increase the change of said frequency in one directionmore than in the other direction until the frequency of the saidgenerator has been changed to produce maximum output of said generator.

15. In an electrical power generating system in which the power outputof a generator has a maximum value at a given frequency and is less asits frequency varies from that value, the method which comprisescyclically varying the speed of said generator first in one directionand then in the other direction to vary its frequency, detecting thechanges in power with said changes in frequency, and in response to therelative changes in power with said changes of frequency, modifying saidcyclical variation of speed of said generator to produce a greaterchange in the direction that increases its power output than in thedirection that decreases its power output until maximum power outputfrom the generator is attained.

16. In a well-drilling system in which the resonant frequency of avibratory transducer changes during penetration of subsurface strata,resulting in a deviation between the frequency of the driving currentand the resonant frequency with a resultant decrease of the vibratoryforces, the method of compensating for changes in said resonantfrequency by varying the speed of the supply generator in mannercomprising cyclically varying the speed of said generator first in onedirection and then in the other direction to vary its frequency,detecting any in crease or decrease in power with said changes infrequency, and in response to the relative changes in power with saidchanges of frequency, modifying said cyclical variation of speed of saidgenerator to produce a greater change in the direction that increasesits power output than in the direction that decreases its power outputuntil maximum power output from the generator is attained.

References Cited in the file of this patent UNITED STATES PATENTS1,621,280 Roucka Mar. 15, 1927 1,843,299 Pierce Feb. 2, 1932 1,966,446Hayes July 17, 1934 2,068,577 Stratton Jan. 19, 1937 2,095,120 Belfilset a1. Oct. 5, 1937

